Polyester films having high intrinsic viscosity have been conventionally used to reduce oligomers, and terminal blocking agents have been used to improve hydrolysis resistance of the polyester films.
Although various heat-resistant films have been used to simultaneously satisfy the above properties, the films still have some problems, such as increased cost, difficult set assembly, unsatisfactory stability after set, and insufficient machinability and processability.
Japanese Patent Application Laid-Open No. 9-100363 discloses a heat-resistant, low-dielectric plastic insulating film having voids and a dielectric constant lower than a particular value. The film has a low dielectric constant to reduce leakage power dissipation in an insulating section caused by high-frequency trends of instruments. Low oligomer contents and ready set assembly when the film is used in electrical insulation, however, are not taken into consideration, and thus the film is not durable in practice. Japanese Patent Application Laid-Open No. 5-194773 discloses a polyester film having a specified apparent density and a tensile modulus, but use of the film in electrical insulation has not been suggested.
These electrical insulating films have the following problems.
(1) In view of global environmental protection, refrigeration media and oils causing lower levels of environmental pollution are being used. Further reduction in oligomers have been awaited in order to achieve this. PA1 (2) Use of only heat-resistant films in refrigeration medium systems results in increased costs. PA1 (3) When films are used in insulation of motors, they have insufficient machinability and processability (in thermoforming and slit bending processing), insufficient stability after set assembly (such as displacement and being loose), and insufficient visibility. PA1 (4) Changes in refrigeration media and oils causes increased leakage of current. PA1 0.6 percent by weight or more: C PA1 0.35 percent by weight or less: A PA1 an intermediate: B PA1 A: The shape did not change. PA1 B: The shape changed slightly, but was near that of the original. PA1 C: The cylinder was completely deformed. PA1 A: Leakage current of 0.8 mA or less PA1 B: Leakage current of 0.8 to 1 mA PA1 C: Leakage current of 1 mA or more
It is an object of the present invention to solve these problems.